Natural product sugar attachments dictate the targeting, biological activity and/or pharmacology of the parent natural product and the alteration of natural product glycosylation patterns has been validated for the generation and/or optimization of natural product-based therapeutics. Yet, simple effective methods to alter sugars appended to natural products are currently lacking. The first phase of this study (RO1 AI52218, years 1-5) led to the development and implementation of a remarkably successful enzymatic strategy to exchange natural product sugars with diverse D-sugar arrays and also provided fundamental information regarding two critical, but poorly understood, enzyme classes (anomeric sugar kinases and sugar-1- phosphate nucleotidylyltransferases). The proposed second phase of this project (years 6-10) is designed to specifically expand this program toward complex natural product L-glycosides. Model GTs have been selected to enable unique studies to probe glycosyltransferase (GT) mechanism with a particular focus upon our recent discovery of the facile reversibility of GT-catalyzed reactions. The enzymes selected - E. coli galK-encoded galactokinase (GalK), Salmonella rmlA-encoded alpha-D-glucose-1-phosphate thymidylyltransferase (RmlA) and a set of GTs which act upon macrolides (MegD1, TylCV and OleD) or nonribosomal peptides (GtfD and GtfE) - are all models for carbohydrate processing reactions ubiquitous in nature and relevant to human disease. Among the many advantages of the targeted natural product scaffolds within this study, differential glycosylation of macrolides and nonribosomal peptides present metabolites with markedly unique antibiotic, antiviral and antiparasitic properties. Cumulatively, the studies described herein will extend our understanding of reactions catalyzed by essential carbohydrate modifying enzymes and offer unprecedented access to uniquely bioactive natural product libraries not readily accessible via conventional organic synthesis or in vivo pathway engineering.